This invention relates to improved liquid scintillation counters. In a particular aspect the invention pertains to improved sample vessels for liquid scintillation counters. In another aspect the invention contemplates sealing means for such sample vessels.
Scintillation counters have become important instruments for studying nuclear radiation. Such nuclear radiation can be made up of energetic particles possessing mass and charge, such as alpha, or beta radiation; of particles having mass and no charge, such as neutrons; or of particles having no mass and no charge, such as gamma radiation.
All of these forms of nuclear radiation interact with matter at the atomic level. This interaction results in disturbing the electron level of an atom with which the radiation particle comes in contact. The measurement of this interaction with matter is the principle around which all radiation counters are designed. Scintillation counters are designed so that the nuclear radiation interacts on a phosphor material. When the phosphor is disturbed it releases some of its disturbed or excited energy as photons. A photosensing device placed in close contact with the phosphor can detect the emitted photons and convert the photon energy to an electrical pulse.
There are two types of scintillation counters presently in use. One is used to measure gamma radiation. This form of radiation having no mass or charge does not readily react with matter. To enhance the interaction phosphors are utilized made up of inorganic crystals of high atomic number atoms, such as sodium or cesium iodide activated with thallium. The second type of counter is used to detect radiation having both mass and energy such as alpha and beta particles. A liquid phosphor in which the radioisotope is dissolved or suspended has been found to be the most effective means for detection. This form of counter is referred to as a liquid scintillation counter after the use of the liquid phosphor.
In liquid scintillation counting a beta or alpha radioisotope is placed in an aromatic solvent into which an organic phosphor is dissolved, the solvent and phosphor collectively being known as a cocktail. The cocktail container or sample vessel can be constructed of glass, pyrex, plastic, quartz and the like having good optical properties, and this sample vessel is referred to as a vial. The vial containing the radioisotope and cocktail is placed in a light-tight well opposite a photosensing device, or between two photosensing devices such as photomultiplier tubes, photodiodes and similar light detecting devices. On decay of the radioisotope in the vial, radiation particles emitted dissipate their ionizing energy into the molecules of the aromatic solvent surrounding the radioisotope. The radiation particles transfer their energy to the solvent molecules, which in turn transfer the energy to the phosphor. The phosphor receiving the transferred energy converts it into photoenergy which is emitted as photons through the walls of the vial. The photons activate the one or more photosensing devices. The photosensing devices, detecting the photon bursts originating from the radiation event, convert this photon energy to electrical pulses which record that a radioactive decay event has occurred within the vial.
Liquid scintillation counters are now the instruments of choice for use in beta decay counting and to a lesser degree, alpha counting, and they have even been modified to measure gamma, and both beta and gamma emissions of radioactive samples. While preferred, this mode of counting nevertheless still possesses certain disadvantages. Two of the main problems with liquid scintillation counters have been their inability to effect high photon detection from within the counting vial, particularly when using radioactive labelled material of photon adsorbing qualities, i.e., quenched samples, and their inability to retain volatile cocktail solvents within the instrument for relatively long periods -- hours to days -- frequently required for sample preparation and counting. Consequently, a need exists for a liquid scintillation vial possessing a more efficient means of emitting photons, preferably one which enhances photon emission, and a vial in which loss of volatile cocktail solvents is prevented. In accordance with this invention a liquid scintillation vial is provided having these improved efficiences.